Road Cut

Prospect, Inactive

Commodities and mineralogy

Main commodities Au; Cu
Other commodities Ag
Ore minerals chalcopyrite; pyrite
Gangue minerals calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale SK
Quadrangle map, 1:63,360-scale A-2
Latitude 59.2
Longitude -135.45
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is on the west side of the Chilkat Peninsula, 3.1 miles south of Haines on the Mud Bay Road as determined by mileposts.

Geologic setting

Geologic description

The following description is summarized from Still (1988). The prospect is in a thick metabasalt sequence within 1/4 mile of an ultramafic intrusive. Mineralization is fault-controlled and is in a fault zone, the Road Cut fault that is up to 40 feet thick, strikes N35-40W, and dips steeply to the northeast. The fault zone consists of silicified, brecciated, and sheared metabasalt and locally sheared and brecciated diorite. At the surface, a higher grade zone outcrops over a strike length of 227 feet. Its eastern (upper) margin includes an 0.2- to 3.5-foot-thick quartz-calcite zone with up to 75% combined pyrite and chalcopyrite. Samples from this quartz-calcite zone contained from 0.8 to 32.26 ppm gold. The remainder of the width of the better mineralized zone consists of a shear zone of silicified metabasalt with 0.06% to 3% chalcopyrite, up to 5% pyrite, and gold values of 0.07 to 0.14 ppm. The higher-grade zone at the surface occurs within a 590-foot-long section of lower grade mineralization that is defined by drill holes and extends to a depth of at least 125 feet. This lower grade zone, which is open along strike to the north and south and down dip, ranges in width from 12 to 40 feet and consists of silicified, locally pyritized, brecciated metabasalt and, locally, brecciated diorite. Gold and copper values average less than 0.48 ppm gold and 268 ppm copper. From the descriptions of Still (1988) it appears that mineralization at the Road Cut and Road Cut II (SK005) prospects occurs along fault and shear zones that are subsidiary shears and splits to the nearby Chilkat fault. If so, the age of the mineralization is probably Tertiary or younger since most movement along the Chilkat Fault has probably been post-Mesozoic (Lanphere, 1978). Plafker, Hudson and Silberling (1979) and Plafker and Hudson (1980) cite fossil evidence for a Late Triassic (Karnian) age for the metabasalts on the Chilkat Peninsula. The nearby ultramafic body is probably 108-109 m.y. old based on potassium-argon ages from a pyroxenite at Battery Point (MacKevett and others, 1974).
A sample of a copper-stained zone in greenstone collected a short distance due west of the Road Cut Prospect on the western shore of the Chilkat Peninsula by Winkler and MacKevett (1970) contained more than 9,999 ppm copper.
Geologic map unit (-135.451829180547, 59.1996741259436)
Mineral deposit model Copper sulfides with some precious metal values along a shear zone in metabasalt.
Age of mineralization The age is not well established. It can be no older than the Late Triassic metabasalts that host the deposit (Plafker, Hudson and Silberling, 1979; Plafker and Hudson, 1980). If the structures that localize the mineralization are related to the Chilkat Fault, then the age of the mineralization is probably Tertiary or youner as most movement on the Chilkat Fault has been post-Mesozoic (Lanphere, 1978).
Alteration of deposit Calcification, pyritization, silicification.

Production and reserves

Workings or exploration Work by the U.S. Bureau of Mines in 1986 and 1987 included geologic mapping and sampling; 13 geophysical survey lines (magnetics, radiometrics and electromagnetics) totaling 7,600 feet in length; 7 diamond drill holes totaling 980 feet in length; and 6 trenches up to 6 feet deep and 20 feet long that were dug with mechanized equipment. Several geophysical anomalies that are subparallel to the deposit were drilled approximately 70 and 120 feet east of the surface mineralization but did not intersect significant mineralization (Still, 1988).
Indication of production None
Reserve estimates Still (1988) divides the mineralized area of the prospect into two zones, a higher grade gold-copper zone and a lower grade 'DDH' zone. He estimates a near-surface resource of 700 tons of material with 0.09 ounce of gold per ton, 0.17 ounce of silver per ton, and 0.8% copper in the gold-copper zone. A slightly deeper resource on this zone is estimated to consist of 4,729 tons of material with 0.02 ounces of gold per ton and 0.1% copper. He estimates a resource of 3 million tons of material with 0.008 ounces of gold per ton in the DDH zone.

Additional comments

This prospect is within a mile of the Chilkat State Park.

References

MRDS Number 10308237

References

Adler, K.M., 1986, Geophysical data review in Haines Road Cut Prospect, Skagway A-2 quadrangle, Alaska: Anchorage, Alaska, On-Line Exploration Services, Inc., 3 p. (Unpublished material available at the Juneau Mineral Information Center, U.S. Bureau of Land Management, Juneau, Alaska).
Adler, K.M., and Adler, K.P., 1987, Haines Road Cut Project, 1987 geophysical survey: Anchorage, Alaska, On-Line Exploration Services, Inc., 3 p. (Unpublished material available at the Juneau Mineral Information Center, U.S. Bureau of Land Management, Juneau, Alaska).
Adler, K.M., and Adler, K.P., 1988, Haines Road Cut Project, 1987 geophysical survey; Update: Anchorage, Alaska, On-Line Exploration Services, Inc., 8 p. (Unpublished material available at the Juneau Mineral Information Center, U.S. Bureau of Land Management, Juneau, Alaska).
Kruger, F.G., 1986, Untitled report covering geophysics conducted on the Road Cut prospect: Salisbury and Associates, Inc., 3 p. (Unpublished material available at the Juneau Mineral Information Center, U.S. Bureau of Land Management, Juneau, Alaska).
Lanphere, M.A., 1978, Displacement history of the Denali fault system, Alaska and Canada: Canadian Journal of Earth Sciences, v. 15, p. 817-822
Plafker, George, and Hudson, T., 1980, Regional implications of Upper Triassic metavolcanic and metasedimentary rocks on the Chilkat Peninsula, southeastern Alaska: Canadian Journal of Earth Sciences, v. 17, p. 681-689.
Reporters T.C. Crafford (T. Crafford & Associates, Anchorage)
Last report date 2/4/2001